arXiv subjects
Publications and source records attributed to S. Bandyopadhyay.
We describe a paradigm for measuring a single electron spin in a solid. This is a fundamental problem in condensed matter physics. The technique can be used to read a spin qubit relatively non-invasively in either a spintronic quantum gate or a spintronic quantum memory. The spin reader can be self assembled by simple electrochemical techniques and can be integrated with a quantum gate.
A curious effect is uncovered by calculating the it time evolving probability of reflection of a Gaussian wave packet from a rectangular potential barrier while it is perturbed by reducing its height. A time interval is found during which this probability of reflection is larger (``superarrivals'') than in the unperturbed case. This nonclassical effect can be explained by requiring a wave function to act as a ``field'' through which an action, induced by the perturbation of the boundary condition, propagates at a speed depending upon the rate of reducing the barrier height.
Compared to quantum logic gates, quantum memory has received far less attention. Here, we explore the prognosis for a solid-state, scalable quantum dynamic random access memory (Q-DRAM), where the qubits are encoded by the spin orientations of single quantons in exchange-decoupled quantum dots. We address, in particular, various possibilities for implementing refresh cycles.
In its most orthodox form, Bohr's Complementarity Principle states that a quanton (a quantum system consisting of a Boson or Fermion) can either behave as a particle or as wave, but never simultaneosuly as both. A less orthodox interpretation of this Principle is the ``duality condition'' embodied in a mathematical inequality due to Englert [B-G Englert, Phys. Rev. Lett., Vol. 77, 2154 (1996)] which allows wave and particle attributes to co-exist, but postulates that a stronger manifestation of the particle nature leads to a weaker manifestation of the wave nature and vice versa. In this Letter, we show that some recent {\it welcher weg} ("which path") experiments in interferometers and similar set-ups, that claim to have validated, or invalidated, the Complementarity Principle, actually shed no light on the orthodox interpretation. They may have instead validated the weaker duality condition, but even that is not completely obvious. We propose simple modifications to these experiments which we believe can test the orthodox Complementarity Principle and also shed light on the nature of wavefunction collapse and quantum erasure.
The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.